Field of the Invention
The present invention concerns a gas-blast circuit-breaker including a jacket filled with a pressurized dielectric gas, two cooperating arc contacts, one of which is part of a mobile contact assembly attached to an operating member and adapted to be displaced axially in the jacket between a closed position and an open position, the mobile contact assembly including a first tube carrying the mobile arc contact at its end and a second tube coaxial with the first tube, carrying a mobile permanent contact at its end and delimiting, on respective opposite sides of a ring joining the first tube and the second tube, a gas-blast chamber closed by a gas-blast nozzle and a compression chamber communicating with the gas-blast chamber and closed by a piston.
Patent application 2 696 274 describes a circuit-breaker of this kind.
In the above prior art document, the piston is semi-mobile and means are provided for immobilizing the piston during a first part of the displacement of the mobile contact assembly during opening of the circuit-breaker and means are further provided for entraining the same piston in the axial direction during a second part of the displacement of this assembly.
The means for entraining the mobile contact assembly in the axial direction during this second part of the displacement comprise an entrainment member attached to the mobile contact assembly which entrains a stop attached to the piston. This stop is disposed on the path of the entrainment member. During the first part of the displacement, the piston is held in position by a spring placed between the piston and the entrainment member and pushing on the piston which is held by a ring "abutted" against a fixed member, the ring being attached to a tube in turn attached to the piston. During this first part of the displacement the entrainment member slides in a slot in the tube connecting the piston to the ring until it abuts the end of the slot, thereafter training the piston during the second part of the displacement.
In an arrangement of this kind the spring acting on the semi-mobile piston applies a permanent force to the operating rod and this leads to an increase in the operating energy required.
U.S. Pat. No. 4,556,767 describes a pressurized gas interrupter having a slightly different structure in that there is no second tube but instead an annular piston 12 coupled to a tube 8 carrying a rated current contact 5 and sliding directly in a cylindrical outer housing 14.
However, the above document describes the use of a piston 15 sliding freely between a stop 16 attached to the housing 14 and a compression spring 17.
This arrangement automatically withdraws the free piston as a result of the increase in the pressure in the compression chamber at the time of the opening maneuver and thereby limits the increase in pressure, thereby providing the advantages of the semi-mobile piston of the first document cited in the introduction but without the operating rod being subjected to any additional force.
When the arc contacts have separated and the gas blast has escaped, the pressure falls and the thrust of the compression spring returns the free piston to its position against said fixed front stop.
Unfortunately, with a free piston of this kind, this return movement is accomplished with non-negligible kinetic energy, producing an impact on the fixed front stop and the parts attaching this stop, which can cause breakages.
Accordingly, an aim of the invention is to propose a circuit-breaker having a structure as defined in the introduction incorporating a free piston with its attendant advantages but overcoming the disadvantage cited hereinabove.